filter_incoming_client
    |
   \|/
switch_incoming_client
  |
  +-->  process_client_stanza   (if no to='' or to='' is for server)
  |
  +-->  deliver


filter_incoming_server
    |
   \|/
switch_incoming_server
  |
  +-->  process_server_stanza   (if to='' is for server)
  |
  +-->  deliver   ( same as earlier deliver )


pre_stanza_write  (before a stanza gets written to client)




Roster
----------------------------------------------------------------------------
RosterGet ($conn, $cb)
   where $cb supports methods:
      ->set_roster(DJabberd::Roster)
      ->decline

Users subclasses DJabberd::RosterStorage can override get_roster, which gets:

     my ($plugininstance, $cb, $conn, $jid) = @_;

(where $jid is just the $conn->bound_jid)